1. Field of the Invention
The present invention relates to wellbore completion. More particularly, the invention relates to an apparatus and method for attaching and sealing two tubulars. More particularly still, the invention relates to maintaining a seal at the connection point between the two tubulars during the expansion operation.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed, and the wellbore is typically lined with a string of steel pipe called casing. The casing provides support to the wellbore and facilitates the isolation of certain areas of the wellbore adjacent hydrocarbon bearing formations. The casing typically extends down the wellbore from the surface of the well to a designated depth. An annular area is thus defined between the outside of the casing and the earth formation. This annular area is filled with cement to permanently set the casing in the wellbore and to facilitate the isolation of production zones and fluids at different depths within the wellbore.
It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The well is then drilled to a second designated depth, and a second string of casing, or liner, is run into the well to a depth whereby the upper portion of the second liner overlaps the lower portion of the first string of casing. The second liner string is then fixed or hung in the wellbore, usually by some mechanical slip mechanism well-known in the art, and cemented. This process is typically repeated with additional casing strings until the well has been drilled to total depth.
Operators are continually searching for means to improve functionality and reduce costs in their drilling operations. This has been achieved in the past in relatively small increments by such things as discrete technological improvements and novel contracting strategies. It is inevitable that these incremental gains are diminishing. The industry needs a radical “next step.” The monobore well is this step.
Monobore wells have a uniform through-bore from the surface casing to the production zones. Today, wells are constructed by progressively securing the borehole with several intermediate casings before the production casing is run. Monobore well technology will eliminate these intermediate casing strings through the use of expandable liners. Monobore wells consist of a sequence of expandable liners that are run through the existing casing, then expanded to achieve the same post-expansion through-bore. A monobore well features each progressive borehole section being cased without a reduction of casing size.
Many conventional wells drilled today consist of a 26-inch or 30-inch conductor and 20-inch or 18 5.8-inch surface casing (or similar sizes), and have several intermediate casings before a 9 5.8-inch production casing is run. The monobore well offers the advantage of being able to start with a much smaller surface casing but still resulting in a 9 5.8-inch production casing. Because top-hole sizes are reduced, less drilling fluid is required and fewer cuttings are created which means less cleaning of the cuttings, and the environmental problem of drilling waste disposal is reduced. Also, with a smaller surface casing size (and only one other actual casing string), the wellhead can be simplified, as can the BOP'S (blow out protectors) and risers. Many drilling plans include a contingency casing or liner to allow for problem zones. Careful planning of a monobore well enables problem zones to be secured without sacrificing a casing size in the well design. Additionally, running expandable liners instead of long casing strings will result in valuable time savings.
Generally, expandable liners for a monobore well or a conventional well are constructed of 38-foot long tubulars. Each expandable liner includes a female thread at the top and a male thread at the bottom. Typically, the top and bottom threaded portions are expandable in the same manner as the expandable liner. A series of expandable liners are commonly screwed together as they are lowered in the wellbore. As the female thread is connected with the male thread, a metal-to-metal seal is created. Additionally, two external O-rings may be disposed on the threads between the two expandable liners to enhance the sealing effect between the male and female thread.
After the entire length of expandable liner is deployed into the wellbore, the expandable liners are typically expanded by the use of a cone-shaped mandrel or by an expander tool. The expander tool is generally run into the wellbore on a tubular string and includes expandable, fluid actuated members disposed on a body. There are a number of advantages to expanding solids using a rotary tool as the expander tool, rather than existing methods involving driving a fixed cone through the pipe. For example, the process is controllable, thereby allowing expansion to be initiated when and wherever required. If necessary, the expander tool can be recovered in a mid-expansion position, and redeployed to continue the operation. Further, the rollers reduce the friction between the tool and the pipe, which allows the expandable liners to be expanded from the top down.
While expanding expandable liners in a wellbore offers obvious advantages, there are problems associated with using the technology. By plastically deforming the expandable liner, the cross-sectional thickness of the expandable liner is necessarily reduced. As a result, the tight metal-to-metal seal created between the female thread and the male thread becomes slack, thereby jeopardizing the seal at the threaded connection. Simply increasing the initial cross-sectional thickness of the expandable liner to compensate for the reduced wall thickness after expansion results in an increase in the amount of force needed to expand the expandable liner.
A need, therefore, exists for an expandable liner connection between two expandable tubulars that maintains its rigidity after the expansion process. There is a further need for an expandable liner connection providing an effective seal at the threaded connection between the expandable liner strings. There is yet a further need for an effective method for ensuring wellbore fluid will not leak through the expandable liners at the threaded connection.